I have been using a tried-and-true method for carbing/serving my beers that required 2.5 volumes of C02. Using the multitude of carbonation charts out there, I have had my kegs at 40 degrees and set my regulator to 12 psi for 7-10 days. I then pour through 6 feet of 3/16" inside diameter hose using the same setting of 12 psi. Works awesome and have had no issues.

Well, now I've done a Belgian Dubbel and want it to be carbed higher. According to the chart, for 3.5 volumes, I set my psi at 24. What do I then do to serve? Do I purge the keg and then set the serving pressure to 12 psi? Do I leave it at 24 and add another 6 feet of hose (total of 12)? I'm just a little confused on the increased psi to 24 as when I had first starting kegging, I would accidentally set my psi to 25 for a week and I had nothing but foam until I learned the patient set-it-and-forget-it method of 12 psi to carb and 12 psi to serve.

I'm wondering if there perhaps isn't any benefit to simply investing in some longer hose and leave the psi at the carbing pressure? Is there the possibility of the beer losing carbonation by purging and then setting it to a lower pressure for serving?

I carb my club soda at 45-50psi for bottling over syrup using a BMBF at the same pressure.

You will lose carbonation if you serve at a lower pressure than carbing. If you consume the entire keg in a shorter period of time than it took to carb, you won't notice, but you will lose carbonation.

If you carb at 24 psi, then set it at 12 psi, your beer will equilibrate over time to somewhere in between 12 and 24 psi depending on temperature. The less headspace you have, the more likely it will equilibrate closer to the carbonation pressure.